During the construction of oil and gas wells a borehole is drilled into the ground. A string of casing is then lowered down the borehole and the annular space between the casing and the borehole filled with cement.
It is important to ensure that the casing is held centrally in the borehole during cementation and it is usual to provide the casing with a plurality of centralizers which act between the casing and the borehole.
Various types of centralizer are known. One type comprises two annular bands which are spaced apart by a plurality of longitudinally extending members. These members can comprise spring bows or rigid members. They share the common feature that, in use, the members extend parallel to the longitudinal axis of the casing on which they are mounted.
Another type of centralizer, often referred to as a "solid centralizer", comprises a casting having a solid central section on which are formed a plurality of fins. In some embodiments the fins are disposed so that, in use, they extend parallel to the longitudinal axis of the casing on which they are mounted. In other embodiments the fins are disposed at an angle so that, in use, when circulating fluid is pumped through the annular space between the casing and the borehole prior to introducing the cement the circulating fluid is encouraged to swirl around the casing. In many situations this also enhances the distribution of cement when it is subsequently injected into the annular space.